1. Field of the Invention
The present invention relates to a composite ceramic material which comprises a matrix of ceramic particles and particles of a different ceramic having a lower sintering temperature than that of the matrix ceramic, the particles of the different ceramic being dispersed in the matrix of ceramic particles, and a method of manufacturing such a composite ceramic material.
2. Description of the Prior Art
As is well known in the art, powders of two or more different ceramics may be mixed and sintered at different sintering temperatures, thereby producing a complex ceramic which exhibits the characteristics of both the ceramics which are mixed.
According to one known method, silicon carbide which is sintered at about 2000.degree. C. and silicon nitride which is sintered in a temperature range from 1700.degree. C. to 1850.degree. C. are mixed together, and the mixture is sintered. The sintered composite ceramic comprises minute particles of silicon carbide which are dispersed in the matrix of silicon nitride. The composite ceramic has a high mechanical strength possessed by silicon nitride, but exhibits a low thermal conductivity of 0.04 cal/cm.sec. .degree. C., which is much lower than the thermal conductivity of 0.07 cal/cm.sec. .degree. C. possessed by silicon nitride itself. It is known that if a composite ceramic is to have a high heat insulation capability so that it can be used as a material for a heat-insulated engine component, then zirconium oxide (hereinafter referred to as "zirconia") may be added.
According to another method, ceramic fibers of silicon carbide are impregnated with silicon nitride by chemical vapor impregnation (CVI), thereby producing composite ceramic fibers with a silicon nitride matrix.
With the this method, the dispersion of silicon carbide is based on the difference between the sintering temperatures of silicon carbide and silicon nitride. Because the sintering temperature of silicon nitride is lower than the sintering temperature of silicon carbide, when silicon nitride and silicon carbide are sintered together, particles of silicon carbide are trapped in particles of silicon nitride. As a result of applying this method, a composite ceramic material comprising a silicon nitride matrix with silicon carbide therein is produced.
Thus, according, to this method the sintering temperature of the ceramic particles to be dispersed must be higher than that of the matrix ceramic, i.e., a ceramic whose sintering temperature is lower than the sintering temperature of a matrix ceramic. Accordingly, this process of making composite ceramic materials can not be applied to ceramics which have sintering temperatures than the sintering temperatures of the matrix materials cannot be dispersed in the matrix ceramic. For example, in conventional sintered composite ceramics of silicon nitride and zirconia since the sintering temperature of zirconia is lower than that of silicon nitride, zirconia exists in the grain boundary of silicon nitride and it is impossible to disperse fine particles of zirconia in a silicon nitride matris. The mechanical strength of the sintered mixture is thus much lower than that of a sintered body formed solely of silicon nitride because the grain boundary can be easily broken due to the presence of zirconia in the grain boundary. Although impossible by this method, in order to lower the thermal conductivity of silicon nitride while maintaining its mechanical strength, it would be better to disperse zirconia in particles of silicon nitride. A second process of manufacturing composite ceramics employs ceramic fibers of silicon carbon that are impregnated with silicon nitride by chemical vapor impregnation (CVI), so that the silicon carbide fibers are incorporated in a silicon nitride matrix.
This second method requires a long period of time to carry out its steps and is costly when applied to bulk ceramics. Therefore, the cost of a composite ceramic component produced by this method is high.